听力原文

The cave river's course is channelled by the beds of limestone. A weakness in the rock can allow the river to increase its gradient and flow-rate, providing a real challenge for the cave explorers.
石灰岩河床将洞穴的河道分割成千沟万壑。石灰岩薄弱处被河水冲刷差距的陡坡处水量激增，为洞穴探险带来相当的挑战。
The downward rush is halted when the water table is reached. Here the slow-flowing river carves tunnels with a more rounded profile.
水流到达地下水位后便停止下流。这里缓缓流动的河流切割出一条圆形隧道。

重点讲解

重点讲解：
1. allow sb. to do sth.
允许；容许；准许某人做某事；
eg. Live events allow enthusiasts to meet in person and showcase their skills.
现场活动给了那些狂热爱好者面对面展示技能的机会。
eg. Do not allow your device to connect to unknown wireless networks.
不要让设备连接到陌生的无线网络。
eg. These glasses allow you to see the film in three-D.
这副眼镜让你能看立体电影。
2. provide sth. for sb.
为某人提供某物；
eg. The hotel provides a reservation of tickets for its residents.
这家旅馆为旅客提供订票服务。
eg. Suburbs sprawled out to provide homes for the newly prosperous.
市郊无规划地向外扩展以为新近富起来的人提供住房。
eg. He was desperate for work to provide food for his children.
为了孩子们有饭吃，他拼命找工作。